Pro-Russian rebels released dozens of captive Ukrainian troops Sunday as part of a ceasefire deal .

Ukrainian President Petro Poroshenko said in a Twitter post that 73 troops were released in Donetsk .

But despite the ceasefire , violence continued to flare in the volatile region . As shelling rocked the city throughout the day Sunday , local officials described the situation as `` critical . ''

Six civilians died and 15 were injured as the result of shelling , Donetsk 's city office said on Monday .

Poroshenko spoke with German Chancellor Angela Merkel over concerns that the ceasefire deal -- brokered earlier this month -- is being violated . They `` agreed to make further efforts to settle the situation peacefully , '' Poroshenko 's office said in a statement .

In an interview with TV Tsentr pm Saturday , Russian Foreign Minister Sergey Lavrov said the truce agreed upon in Minsk , Belarus , appeared to be holding generally and that Moscow , at least , is ready to work toward a long-term peace .

`` Sporadic exchanges of fire occur on both sides , but the process of establishing -LRB- a -RRB- durable peace is still in progress , '' he said , according to parts of that interview published by the state-run Itar-Tass news agency .

Which Russian companies have been hit ?

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Shelling kills 6 civilians in Donetsk , city officials say

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73 Ukrainian troops are released by pro-Russian rebels in Donetsk

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President Petro Poroshenko talks about ceasefire violations with German leader

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As shelling continues , officials in Donetsk describe the situation as `` critical ''